This invention relates to the removal of harmonic currents from power distribution lines. More particularly, this invention relates to an active power filter that cancels the harmonic currents in the neutral wire of a three-phase, four-wire electric distribution system.
A typical low voltage electric distribution system in the U.S. consists of a 208/120 volt three-phase, four-wire system. These systems are widely employed to distribute electric energy to loads within buildings and plants. In such systems, under normal operating conditions with nearly balanced loads, the current in the neutral wire is small and does not exceed twenty percent of the normal phase current magnitude.
However, in recent years, the use of solid-state power conversion equipment to service loads has greatly increased. Typical loads now connected to the low voltage three-phase, four-wire system include adjustable speed heating ventilation and air-conditioning (HVAC) systems, fluorescent lighting circuits with conventional and electronic ballasts, computer systems, and other sensitive electronic loads. All of these loads employ switched mode type power electronic converters which draw excessive harmonic currents, a significant portion being the third harmonic (180 Hz) component. Further, saturated iron cored inductive ballasts as well as electronic ballasts in fluorescent lighting circuits also contribute to third harmonic currents. The third harmonic and odd multiples of the third harmonic (i.e. 9th, 15th, etc.) do not cancel each other in the neutral wire but in fact are additive. These harmonic currents flow back to the power system through the neutral wire.
Resultantly, the neutral current in the modern three-phase, four-wire distribution systems is often excessive. In some installations, the neutral current magnitude exceeds the magnitude of the phase currents. The excessive neutral current often causes wiring failures, wire overheating, and damages distribution transformers. The excessive neutral current also causes noise due to thermal cycling and excessive neutral to ground voltage drop, both of which can damage sensitive electronic equipment.
Prior remedies to the excessive neutral current problem included derating transformers, using separate neutral wires for non-linear loads, tripping phase wires when the neutral current exceeded a threshold level, and simply increasing the capacity of the neutral wire. However, these solutions inevitably failed. Derating distribution transformer ratings temporarily solved the problem but resulted in a lower efficiency of operation and required that additional units be installed to serve existing load. Further, installing separate neutral wires to serve non-linear loads and increasing the size of the neutral wires is often non-feasible, always expensive, and does not eliminate the underlying problem.
Another proposed solution, included selectively filtering harmonic components of the neutral current wire using passive filters. However, since passive filters are dependent upon the particular frequency, are expensive, and are typically inefficient, they also were unsatisfactory. Another proposed solution included the use of nonstandard zigzag transformer connections to allow circulation of triple harmonics in the secondary windings of the transformer. The technique, even though somewhat effect, was sensitive to the system impedance and often required a choke to isolate the neutrals of the power system from the zigzag transformer. However, once installed, the choke caused the power system neutral voltage to float.